1. Field
Example embodiments relate to brushless-motor driving control devices, image reading apparatuses, and image forming apparatuses.
2. Discussion of the Related Art
In conventional brushless DC motors, typically, rotation is controlled by detecting the position of a rotor using sensors, for example, Hall elements.
In the case of a three-phase brushless DC motor, one Hall element is needed for each phase, so that three Hall elements are needed in total.
In another conventional motor driving method, called the sensorless DC motor driving method, rotation of a brushless DC motor is controlled on the basis of induced voltages that occur on coils of the motor without using sensors, for example, Hall elements.
The sensorless DC brushless motor driving method is already used in large-scale machines in which a considerable amount of heat is generated so that it is not feasible to use sensors, for example, Hall elements.
In various types of electronic equipment in which a motor is used to generate a driving force, the sensorless DC brushless motor driving method may be used to reduce cost.
In the sensorless DC brushless motor driving method, the position of the rotor may be detected on the basis of inducted voltages that occur on the coils of the motor.
For example, at the time of activation of the motor, induced voltages do not substantially occur on the coils of the motor, so that excitation currents are sequentially supplied to the coils at predetermined or desired timings so that the motor is forced into an active state.
That is, during activation of the motor, open-loop control is exercised.
After the rotation rate of the motor increases so that sufficient induced voltages occur on the coils, switching timings for excitation of the coils are calculated on the basis of induced voltage signals, and the sensorless DC brushless motor may be driven according to the switching timings.
During an early period of activation, currents may be forced to flow through coils of individual phases using angular-position detecting signals of a rotary member that is driven by a driving force generated by the motor.
When the motor is activated, the rotor is pulled to three predetermined or desired positions to determine references for timings of switching currents for the individual phases on the basis of angular-position detection signals.
After the references are determined, the positions of magnetic poles of the rotor may be detected on the basis of angular-position detection signals so that the motor may be drive without using Hall elements or induced voltages.
According to the sensorless brushless DC motor driving method described above, in which the position of the rotor is detected using an induced voltage, an induced voltage is not obtained when the motor is stopped.
Thus, during a certain period after activation of the motor, until the induced voltage reaches a detectable level, excitation currents are sequentially supplied to the coils of the motor at predetermined or desired timings, thereby activating the sensorless brushless DC motor.
This method of controlling activation is effective for devices with stable load or devices in which problems do not occur even when the rotation rate becomes too high.
However, when the existing sensorless brushless DC motor driving method is used in a laser beam printer, a copying machine, or the like, the following problems may arise.
First, since the load of the motor is not constant at the time of activation depending on the status or amount of toner in the toner cartridge, a sufficient torque may not be generated depending on the position of the rotor at the time of activation, so that activation fails.
Second, since open-loop control is exercised at the time of activation, rotation of the motor at an excessively high rotation rate might cause degradation of the photosensitive drum.
In order to overcome these problems, according to Japanese Unexamined Patent Application Publication No. 2003-79184, a first detector detects a rotation position of a rotor of a motor, and a second detector detects a rotation position of the rotor on the basis of an inducted voltage that occurs on a coil when a current is not flowing through the coil.
When the motor is activated, driving control of the motor is started on the basis of the result of detection by the first detector. Then, at predetermined or desired timings, driving control of the motor is switched on the basis of the results of detection by the first detector and the second detector.
Until it becomes possible to detect the rotation position on the basis of an induced voltage, the first detector detects regularly spaced timing marks formed on a detection target that rotates together with the rotor to count the number of pulses output successively, excitation currents are forcibly supplied sequentially to the coils of the motor when the number of pulses counted reaches a predetermined or desired number.
During this period, open-loop control is exercised, e.g., the rotation rate is not controlled on the basis of detection of the angular velocity of the motor.
According to Japanese Patent No. 3690296, the rotor of the motor is pulled to three predetermined or desired angular positions, the angular positions are defined as corresponding to timing of switching currents for individual phases of the motor on the basis of signals from an angular-position detector provided on a driven member that is driven by a driving force transmitted from the sensorless motor.
After the setting, switching among a plurality of excitation phases is controlled according to signals from the angular-position detector.
However, according to the related art disclosed in Japanese Unexamined Patent Application Publication No. 2003-79184, until the rotation rate of the motor becomes sufficiently high so that an induced voltage can be used, it is difficult to drive the sensorless DC brushless motor by calculating timing of switching among excitation phases on the basis of induced voltage signals.
Since open-loop control is exercised, e.g., the rotation rate is not controlled on the basis of detection of angular velocity of rotation, a delay occurs when a feedback control mode is entered.
When a delay occurs in activation, in the case of a laser beam printer, a copying machine, or the like, the time of starting the first print job is delayed.
Even when activation takes a considerable time so that the temperature is raised sufficiently to allow fixing, it is desired to reduce it takes from receipt of a job instruction to output of a job product.
Thus, it is needed to reduce time for activation of the driving system. An analog circuit for detecting induced voltages is needed. This results in increased cost.
According to the related art disclosed in Japanese Patent No. 3690296, it is possible to start driving the motor without detecting induced voltages. However, at the time of activation, it is needed to recognize three predetermined or desired angular positions, so that a delay occurs in activation.
Error occurs in the predetermined or desired rotation angles due to the effect of friction torque, error occurs in estimated positions of the magnetic poles of the rotor relative to the actual positions thereof. This causes unstable operation.
When power is turned off, it is not allowed to recognize the phases of the magnetic poles accurately.
Thus, when power is turned on again, the rotor may be pulled to the predetermined or desired angular positions. This causes a delay in activation.